A typical aircraft braking application makes use of hydraulic adjusters that apply linear force to a brake stack. Regulated hydraulic pressure is applied to force an actuating piston outward, compressing the rotating discs and the stationary discs in the brake stack. The resulting friction causes a braking action on the wheel and tire assembly. When the hydraulic pressure is relieved, a retracting spring forces the actuating piston to retract linearly into the housing chamber leaving a predetermined running clearance between the rotating and stationary discs.
Recently, however, some aircraft braking systems are translating from hydraulic to electric actuation by making use of EMAs. This type of actuator relies on a motor and controller to create the desired amount of retraction of the piston. Alternately, if the motor loses power, the reverse efficiency of the system must be sufficient to permit back pressure from the brake stack to back drive the piston to a minimum desired clearance position to prevent a dragging brake and unsafe aircraft operation.